Diffraction Limited Optics

Hi Guys and Gals,

Can anyone tell me in layman’s term what is “Diffraction Limited Optics”? I’m seeing the term used in advertising and I’ve read some of Harold Suiter’s book but I’m still not sure what I’m getting when I purchase a scope with “Diffraction Limited Optics”. Optical theory isn’t my strongest suite so take it easy on me, please.

Thanks,
Dave

Hi David,
All telescopes have an inherent limitation to their angular resolution due to the diffraction of light at the telescope's aperture. When a continuum of wave components pass through an aperture, the superposition of these components result in a pattern of constructive and destructive interference. For astronomical instruments, the incoming light is approximately a plane wave since the source of the light is so far away. In this far-field limit, Fraunhofer diffraction occurs and the pattern projected onto the focal plane of the telescope will have little resemblance to the aperture. (Hecht, 1987; Heald & Marion, 1995).

Said of an optical system that meets the Rayleigh criterion, or, put another way, that can bring light to a focus at the theoretical minimum of the Airy disk. To be diffraction-limited there must be no more than ¼ wavelength of light between wavefronts at the focal point. Modern optical systems can exceed this theoretical design limit, but it remains a standard

I hope these two definitions explain it well enough.

For whatever its worth here is one more take on the definition of diffraction limited optics; Personally I do not consider Rayleigh 1/4 wave criteria applicable to most well corrected scopes that are availably to amateurs today, the definition simply implies that telescopes wavefront errors are small enough that diffraction effects limits its performance, in layman's term this means optics are as good as it gets.
The 19th century Rayleigh 1/4 wave criterion applies only to unobstructed telescopes, telescopes with central obstruction require much higher quality to deliver performance that a 1/4 wave refractor is capable of.
As for Ceravolo experiment and its conclusions, most participants in that experiment had a very short time to guess the optical quality of those 6" reflectors, there are many variables that can affect the conclusions including seeing and persons unfamiliarity with that particular type instrument.
I can personally tell the difference between 1/4, 1/10 or even better wavefronts up to 6" aperture simply because 6" and smaller apertures, when seeing allows, do reach their maximum potential frequently, on larger apertures it becomes extremely difficult if not impossible to see the difference due to seeing limitations.

Thanks,
Vahe

David Pope said:

Can anyone tell me in layman’s term what is “Diffraction Limited Optics”?

"Diffraction limited" is somewhat vague expression, that no one seem to be able to trace direct origin from. Conventionally, it
is accepted as reffering to wavefront quality of 0.8 Strehl (1/13.4 wave RMS, or 1/4 wave p-v of spherical aberration), or better. In effect, "diffraction limited" without further specification covers anything from good to excellent optics.
Difference within can be noticeable, but not significant to the
extent that would amount to a gross inferiority/superiority.
The fact is that actual quality level is bellow "diffraction-limited" for all but smallest apertures (<4"), regardless of
the optical quality. This is due to the seeing and other error sources. Better quality optics helps to keep actual quality higher: it does matter if that actual quality is, say, 1/3 wave vs. 1/2.5 wave p-v level.

One has to keep in mind that "diffraction limited" optics with
obstructed telescopes doesn't necessarily mean "diffraction-limited" performance. Due to the effect of c.obstruction, "diffraction-limited" level for obstructed apertures requires better optics, as approximated with the Strehl given by 0.8/(1-c^2)^2, "c" being the relative size of
c.obstruction (in units of aperture). So a 25% obstructed telescope (c=0.25) would need optics of 0.91 Strehl (1/6 wave p-v) to perform at a true 0.8 Strehl level. Obstructions greater than 32% make it impossible to reach true "diffraction-limited" minimum, as defined above.

Vlad

Dave,

I agree with most of the discussion that has been going on here--But you will notice that the thread has broken into two areas. The first is your original question--What does "diffraction limited mean?"--as in "explain the concept." The second part of the thread is "What number do we assign to "diffraction limited?" As in "1/4 wave, 1/8 wave--what?"

To answer the first part of this--this is my take:

There are four things that will limit viewing:

1. Viewer Limitations: These are physical and or experience limitations of the viewer. If you have bad eyes, or not much experience, then you won't be able to see very well no matter what you use.
2. Viewing conditions: If the atmosphere is unstable, or there is a lot of light pollution, or clouds, or the mirrors/lenses are not thermally equalized, or whatever, you won't be able to see very well.
3. The quality of the instruments. And there are perhaps two parts here, a) the optics themselves, and b)how the optics are used. a) the optics themselves include the accuracy of the lens or mirror designs, the production of that lens or mirror or other optics (These are the factors that would earn the optics the designation "diffraction limited or not.). b), How the optics are used, would include the stability of the mount, collimation, cleanliness, and all that of the telescope assembly (These factors would not qualify as a good judge of "diffraction limited," because they depend on the user and the non-optical parts of the assembly, and not the optics themselves.).
4. The physics of optics. And this starts with the fact that light does not in fact travel in straight lines, but instead is "diffracted" around edges, and therefore slightly scattered.

So, with this framework in mind, "diffraction limited" means the physics of optics are in fact the limiting factor in how good the image will be. All the other errors of the optics (color abberations, different focusing points for different parts of the optics, whatever) are smaller than the errors introduced by the fact that light diffracts.

Now, notice that this definition only applies to the optics (Those parts that I listed in part 3A above.).

What is the point at which diffraction limits optics? That is the second part of the discussion in this thread.

I don't know the actual optical physics, but here is what I do know:

The Rayleigh criterion specifies that if the actual wavefront, after reflection, differs from perfection by less than ¼ wavelength of the color of light that the eye is most sensitive (560nm), then the image formed can appear essentially perfect. "After reflection" means the mirror surface error must be no greater than 1/8 wave. The reflection doubles the surface error. This, of course is an ideal situation, and assumes there are no other contributors to image deformation in the light path. (see Suiter, H. R., Star Testing Astronomical Telescopes for the definitive work on image aberration).

Therefore, we talk about a "1/8 wave" mirror or 1/4 wave lens as a good approximation of the Raleigh criterion.

But, of course, if you go back to my earlier exposition of the four limitations on viewing, there are the viewer limitations, the viewing conditions, and half of the instrument conditions that may affect your viewing more than the quality of the optics or diffraction. And that is the source of the controversy on the numbers.

Or at least that is what I think.

Alex

Thanks to all for their input on this discussion. Although the math and physics are a little over my head, I do believe that it has helped me to understand the term a little better.

I’m in the market for a secondary scope to travel with and on nights in which I don’t have much time to bring out the 15” Obsession. And I was noting some ads in which scopes that are selling for $500 advertising Diffraction Limited Optics. Now, I'm a bit of a tinker with mechanical issues and don’t mind modification here and there. But the promise of quality optics in a scope for that price has me raising my eyebrows. I just wanted to clarify the term before jumping on such a “deal”.

Dave